The present invention relates to multilayered infrared reflecting films, and more particularly to polymeric multilayered films which reflect light in the infrared region of the spectrum while transmitting light at substantially all wavelengths in the visible spectrum with an absence of visibly perceived iridescent color.
Coextruded multilayer infrared reflecting films have been made which comprise multiple alternating layers of two polymers with individual layer thicknesses of 300 nanometers or less. Such multilayer films are described, for example, in Alfrey et al, U.S. Pat. No. 3,711,176. When polymers are selected to have a sufficient mismatch in refractive indices, these multilayer films cause constructive interference of light. This results in the film transmitting certain wavelengths of light through the film while reflecting other wavelengths. The multilayer films can be fabricated from relatively inexpensive and commercially available polymer resins having the desired refractive index differences.
The reflection and transmission spectra for a particular film are primarily dependent on the optical thickness of the individual layers, where optical thickness is defined as the product of the actual thickness of the layer times its refractive index. Films can be designed to reflect infrared, visible, or ultraviolet wavelengths of light depending on the optical thickness of the layers. When designed to reflect infrared wavelengths of light, such prior art films also exhibit higher order reflections in the visible range, resulting in an iridescent appearance for the films. The films produced in accordance with the above mentioned Alfrey patent exhibit iridescence and changing colors as the angle of incident light on the film is changed.
For some applications, while reflection of infrared wavelengths is desirable, higher order reflections of visible light are not. For example, infrared reflecting films can be laminated to glass in buildings and automobiles to reduce air conditioning loads. The films may also be laminated to other substantially transparent plastic materials to reflect infrared wavelengths. However, the films must be substantially transparent to visible light so that the vision of those looking through the glass or plastic is not impaired.
It is possible to suppress some higher order reflections in the visible range of the spectrum by proper selection of the optical thickness ratio in two component multilayer films. See, Radford et al, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films," Polymer Engineering and Science. vol. 13, No. 3, May 1973. However, it is not possible to suppress two successive higher order reflections with two component films alone unless somewhat complex three-layer equivalent films (two component films which simulate three components) are used. See, M. Ohmer, "Design of Three-Layer Equivalent Films", J. Optical Soc. of Am., v.68(1), pp. 137-139 (1978).
Schrenk et al, in commonly assigned U.S. Pat. No 5,103,337, teach a three component infrared reflecting film which transmits a majority of visible light. The three component film is capable of suppressing three higher order reflections so that the film appears substantially colorless. However, due to the nature of reflective interference films, the film may still display localized non-uniform spots of iridescent color due to thickness variations or problems with control during the coextrusion process.
Other workers have designed optical coatings comprising layers of three or more metal oxides and halides which are able to suppress certain higher order reflections in the visible spectrum. For example, Thelen, U.S. Pat. No. 3,247,392, describes an optical coating used as a band pass filter reflecting in the infrared and ultraviolet regions of the spectrum. The coating is taught to suppress second and third order reflectance bands. However, the materials used in the fabrication of the coating are metal oxide and halide dielectric materials which must be deposited using complex vacuum deposition techniques. Also, once deposited, the coatings and the substrates to which they are adhered cannot be further shaped or formed without destroying the integrity of the coatings.
Michelotti et al, U.S. Pat. No. 4,971,843, describes a tin or other metal oxide film which is deposited on one surface of a glass sheet and which is of a thickness to produce an infrared reflective effect. However, due to second or higher order effects, the infrared reflecting film also exhibits iridescent color in the visible range. To mask the iridescent color, the patentees teach that a second metal oxide coating is deposited on an opposite surface of the glass which is uniformly reflective over the visible range. However, the use of metal oxide films involves complex deposition procedures to produce the desired products, and further, Michelotti et al require an intervening sheet of glass for the films to adhere to. The Michelotti films cannot be retrofitted onto existing structures.
Wheatley et al, in commonly-assigned copending application Ser. No. 07/629,520, filed Dec. 18, 1990, and entitled "Polymeric Reflective Bodies with Multiple Layer Types", now U.S. Pat. No. 5,126,880, issued Jun. 30, 1992, describes a uniformly reflective all polymer body in which the iridescent effects from optically thin layers are masked. However, the body is not designed to be transparent to visible light.
Accordingly, the need still exists in this art for a multilayered film which reflects light in the infrared region of the spectrum, while transmitting light in the visible region of the spectrum which has substantially no visibly perceived iridescent color, and which is easily produced.